The present invention relates to a method for forming fine pattern of a conjugated polymer film.
Conjugated polymers represented by polyacetylene are expected to be applied to devices such as memory devices, sensors, solar batteries, storage batteries, optical devices, and nonlinear optical devices. Considering application to such photoelectronic devices, to form conjugated polymers into a desired pattern is a technique which is very advantageous for formation of circuits using them. It is known, for example, in U.S. Pat. No. 4,816,383 to form a negative pattern by coating on a substrate a solution containing a polymeric sulfonium salt which is a precursor for poly-p-phenylenevinylene, then irradiating the coat with ultraviolet rays in pattern form and then removing the film of unexposed portion with a solvent. This pattern of precursor can be converted to a conjugated polymer poly-p-phenylenevinylene by heat treatment.
When conjugated polymers are used for memory devices, optical devices and nonlinear optical devices, generally a fine circuit pattern is formed on a flat substrate by photolithography. Photolithography is commonly carried out by mask aligner and stepper which have as light sources bright lines such as i-line (365 nm), h-line (404.7 nm) and g-line (435.8 nm) of high pressure mercury lamp and cost for apparatus is low and performance thereof is stable. In order to obtain finer pattern, irradiation with light of short wavelength is conducted. However, irradiation with ultraviolet rays of short wavelength from light sources such as xenon lamp, xenon mercury lamp, and excimer laser sometimes causes side reactions undesirable for resist materials such as decomposition and crosslinking. Such crosslinking or decomposition damages characteristics of conjugated polymers, for example, cleavage of conjugaged system. Therefore, development of lithographic technique with wavelength which causes no side reactions has been demanded for forming pattern of conjugated polymers. Furthermore, if the popular irradiation device which uses high pressure mercury lamp can be utilized, this is industrially advantageous.
According to the research conducted by the inventors, in case of poly-p-phenylenevinylene, when a polymeric sulfonium salt which is a precursor is irradiated by high pressure mercury lamp, this was insufficiently cured and swollen in a developing solution, so that sufficiently fine pattern was not able to be formed. Furthermore, depending on kind of polymeric sulfonium salt which is a precursor and kind of solvent for development, positive patterns may be formed with irradiation by high pressure mercury lamp, but these positive patterns are inferior in resolution and sufficiently fine patterns cannot be formed.